1. Field of the Invention
This invention relates to the processing of semiconductor wafers. More particularly, this invention relates to improvements in the process of depositing a tungsten layer on a semiconductor wafer.
2. Description of the Related Art
In the formation of integrated circuit structures on and in a semiconductor wafer such as a silicon wafer, one or more metals layers are conventionally used to form conductive interconnects or wiring harnesses to various contacts on the processed wafer. Such metal wiring harnesses or patterned metal layers have, in the past, utilized either aluminum or gold as the metal. From an economical standpoint, aluminum is preferred to gold. However, as the geometries of the integrated circuit structures become smaller and smaller, the use of tungsten metal, instead of aluminum, has become preferred due to the better step coverage and filling of small contact holes or vias by CVD tungsten.
However, the blanket deposition of a layer of tungsten on a semiconductor wafer is not without problems. First of all, the conventional deposition of tungsten at about 350.degree. C. is rather slow, i.e., about 300 Angstroms per minute. Higher rates, e.g., about 1000 Angstroms per minute, are achievable, but only by raising the temperature up to about 500-550.degree. C., which may risk damage to underlying portions of the integrated circuit structure.
Furthermore, the resulting layer of tungsten is not always smooth, resulting in a reflectivity which may be only 20% or less that of a silicon surface, making subsequent patterning of the tungsten layer by photolithography techniques more difficult. Also, the uniformity of the deposition may vary over 1% in thickness across the wafer, as measured by the resistivity.
It would, therefore, be beneficial to provide improvements in the process for the blanket deposition of a tungsten layer on a semiconductor wafer wherein the deposition rate could be accelerated and the reflectivity of the resulting tungsten layer improved, as well as providing a more uniform layer.